Detection of cldn18-arhgap6 fusion gene or cldn18-arhgap26 fusion gene in pancreatic cancer

ABSTRACT

The purpose is to reveal a polynucleotide that is a novel causal gene of pancreatic cancer and thereby provide a method for detecting the polynucleotide or a polypeptide encoded thereby to select a subject positive for the polynucleotide or polypeptide and a method expected to be useful for to identify patients suitable for therapies and a primer set therefor and a kit for detection. In the method, a polynucleotide comprising a fusion point of a part of a CLDN18 gene and an ARHGAP6 gene or a polynucleotide comprising a fusion point of a part of a CLDN18 gene and an ARHGAP26 gene, or a fusion protein encoded thereby is detected. The primer set comprises a sense primer designed for a part encoding CLDN18 and an antisense primer designed for a part encoding ARHGAP6 or a part encoding ARHGAP26.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 16/324,366, which is the U.S. National Stage of PCT/JP2017/028906, filed Aug. 9, 2017, which claims priority to JP 2016-157167, filed Aug. 10, 2016 and JP 2016-221434, filed Nov. 14, 2016.

The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-WEB and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Jun. 1, 2021, is named sequence.txt and is 76,903 bytes.

TECHNICAL FIELD

The present invention relates to the detection of CLDN18-ARHGAP6 fusion gene or CLDN18-ARHGAP26 fusion gene.

BACKGROUND ART

Claudin-18 (CLDN18) gene is located on the long arm of human chromosome 3 and a protein encoded by this gene is a four-transmembrane protein. As to its function, this protein has been expected to contribute to the cellular attachment by tight junction because it has been reported that claudin 1 (CLDN1) and claudin 2 (CLDN2), which belong to the same gene family, each forms complexes with occludin (OCLN), which is also a four-transmembrane protein, to constitute tight junction (J Cell Biol., 1998; 143 (2): 391-401) and that CLDN18 gene-knockout mice had aberrant structure of the tight junction between gastric epithelial cells, and also suffered from gastritis due to leak of gastric juice into the tissue (Gastroenterology, 2012; 142 (2): 292-304). With respect to cancer, it has been reported that the expression level of CLDN18.2, which is one of the splicing variants, was elevated in various cancers (Clin Cancer Res. 2008; 14 (23): 7624-7634), while it has also been reported that gastric cancer patients with high CLDN18 expression levels have better prognoses (Int J Surg. 2014; 12 (2): 156-162).

Rho GTPase activating protein 26 (ARHGAP26) gene, a GTPase activator, is located on the long arm of human chromosome 5 and a protein encoded by this gene is GTPase activating protein having a Rho-GAP domain at the middle. As to its function, it is known that the protein increases GTP hydrolase activity of the small GTPase protein family, in particular, ras homolog family member A (RhoA) and cell division cycle 42 (CDC42) (J Biol Chem., 2000; 275 (49): 38605-38610). With respect to cancer, ARHGAP26 has been found in gastric cancer (PTL 1) and, in particular, its fusion gene with CLDN18 gene has been found in 3% of gastric cancer (NPL 1) and, in particular, 15% of poorly differentiated gastric cancer patients (NPL 2), respectively in primary lesions. In addition, its fusion gene with mixed-lineage leukemia (MLL) gene has been found in leukemia patients (Proc Natl Acad Sci U S A., 2000; 97 (16): 9168-9173, Genes Chromosomes Cancer, 2004; 41 (4): 400-404).

Rho GTPase activating protein 6 (ARHGAP6) gene, a GTPase activator, is located on the short arm of human X chromosome and encodes a GTPase activating protein having a Rho-GAP domain in the middle, like ARHGAP26 described above. As to its function, it is known that the protein also increases, like ARHGAP26, GTP hydrolase activity of the small GTPase protein family, in particular, RhoA (Hum Mol Genet., 2000; 9 (4): 477-488). With respect to cancer, its fusion gene with CLDN18 gene described above was found in primary lesions in patients suffered from poorly differentiated gastric cancer, although with a small percentage (NPL 2).

Currently, the fusion gene of CLDN18 and ARHGAP26 and the fusion gene of CLDN18 and ARHGAP6 have not been reported in any other cancer types except to gastric cancer. For example, the article published in Nature, 2016; 531: 47-52, reported results of mutation analysis in pancreatic cancer specimens, in which they reported that mutations in 32 genes were recurrently detected in pancreatic cancer specimens and pancreatic cancer cell lines (n=456). However, they also reported that any fusion genes were not detected recurrently, and if detected, in just one specimen (“No recurrent fusion events were detected”). Besides, they did not mention about the fusion gene of CLDN18 and ARHGAP26, or the fusion gene of CLDN18 and ARHGAP6.

CITATION LIST Patent Literature

PTL 1: International Publication No. WO 2015/142293

Non Patent Literature

NPL 1: Cell Rep., 2015; 12 (2): 272-285

NPL 2: Nature, 2014; 513 (7517): 202-209

SUMMARY OF INVENTION Technical Problem

The purpose of the present invention is to reveal a polynucleotide that is a novel causal gene for pancreatic cancer, and thereby to provide methods to select subject positive for the polynucleotide or the encoded polypeptide by detecting them, methods expected to be useful to identify patients suitable for therapies, primer sets, probes, probe sets, or kits for the detection therefor.

Solution to Problem

The present inventor found that a polynucleotide comprising a fusion point between a part of CLDN18 gene and a part of ARHGAP6 gene, or a polynucleotide comprising a fusion point between a part of CLDN18 gene and a part of ARHGAP26 gene, can be detected in pancreatic cancer patients (Example 1). Based on the finding, the present inventor constructed a step to detect a polynucleotide comprising a fusion point of the fusion gene, provided primer sets therefor, and enabled to provide a method expected to allow selection of pancreatic cancer patients positive for a fusion gene of CLDN18 gene and ARHGAP6 gene, or a fusion gene of CLDN18 gene and ARHGAP26 gene by detecting a polynucleotide comprising a fusion point of the aforementioned fusion gene.

Accordingly, the present invention relates to the following [1] to [25].

-   [1] A method for selecting a subject positive for a fusion gene of     Claudin-18 (CLDN18) gene and Rho GTPase Activating Protein 6     (ARHGAP6) gene or a subject positive for a fusion gene of Claudin-18     (CLDN18) gene and Rho GTPase Activating Protein 26 (ARHGAP26) gene,     comprising a step of detecting the presence of a polynucleotide     comprising a fusion point of CLDN18 gene and ARHGAP6 gene or a     polynucleotide comprising a fusion point of CLDN18 gene and ARHGAP26     gene in a sample from a subject suspected to have pancreatic cancer     or a subject having pancreatic cancer. -   [2] The method according to [1], wherein the polynucleotide is     polynucleotide encoding a polypeptide of (1) or (2) as follows: -   (1) a polypeptide comprising an amino acid sequence having 90% or     more identity with an amino acid sequence set forth in SEQ ID NO: 2,     4, 6, or 8; -   (2) a polypeptide having an amino acid sequence modified from an     amino acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8 by     deletion, substitution, insertion, and/or addition of 1 to 10 amino     acids. -   [3] The method according to [2], wherein the polypeptide of (1)     or (2) has tumor promoting ability. -   [4] The method according to [2], wherein the polynucleotide is a     polynucleotide encoding a polypeptide consisting of an amino acid     sequence set forth in SEQ ID NO: 2, 4, 6, or 8. -   [5] The method according to [1], wherein the polynucleotide is a     polynucleotide of (3) or (4) as follows: -   (3) a polynucleotide comprising a nucleotide sequence having 90% or     more identity with a nucleotide sequence set forth in SEQ ID NO: 17     or 19; -   (4) a polynucleotide comprising a nucleotide sequence modified from     a nucleotide sequence set forth in SEQ ID NO: 17 or 19 by deletion,     substitution, insertion, and/or addition of 1 to 10 nucleotides. -   [6] The method according to [5], wherein the polynucleotide is a     polynucleotide consisting of a nucleotide sequence set forth in SEQ     ID NO: 17, 18, 19, 20, or 21. -   [7] The method according to any of [1] to [6], further comprising a     step of determining that the subject is a subject positive for a     fusion gene of CLDN18 gene and ARHGAP6 gene or a subject positive     for a fusion gene of CLDN18 gene and ARHGAP26 gene when the presence     of the polynucleotide is detected, and determining that the subject     is not a subject positive for the fusion gene when the presence of     the polynucleotide is not detected. -   [8] The method according to any of [1] to [7], comprising a step of     amplifying a nucleic acid in a sample obtained from the subject     and/or hybridizing a probe with a nucleic acid in a sample obtained     from the subject for detecting the presence of the polynucleotide. -   [9] The method according to [8], comprising a step of amplifying a     nucleic acid in a sample obtained from the subject using a primer     set as follows:

a primer set for detecting the fusion gene of the CLDN18 gene and the ARHGAP6 gene or the fusion gene of the CLDN18 gene and the ARHGAP26 gene, comprising a sense primer designed for a part encoding CLDN18 and an antisense primer designed for a part encoding ARHGAP6 or a part encoding ARHGAP26, wherein the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide under stringent conditions and the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide under stringent conditions.

-   [10] The method according to [9], wherein

the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 1 under stringent conditions, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide consisting of nucleotides at positions 751 to 3087 in SEQ ID NO: 1 under stringent conditions;

the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 3 under stringent conditions, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide consisting of nucleotides at positions 751 to 2460 in SEQ ID NO: 3 under stringent conditions;

the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 5 under stringent conditions, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide consisting of nucleotides at positions 751 to 2088 in SEQ ID NO: 5 under stringent conditions; or

the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 7 under stringent conditions, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide consisting of nucleotides at positions 751 to 1923 in SEQ ID NO: 7 under stringent conditions.

-   [11] The method according to [9] or [10], wherein

the sense primer consists of an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 1 to 750 in SEQ ID NO: 1, and the antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 751 to 3087 in SEQ ID NO: 1;

the sense primer consists of an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 1 to 750 in SEQ ID NO: 3, and the antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 751 to 2460 in SEQ ID NO: 3;

the sense primer consists of an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 1 to 750 in SEQ ID NO: 5, and the antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 751 to 2088 in SEQ ID NO: 5; or

the sense primer consists of an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 1 to 750 in SEQ ID NO: 7, and the antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 751 to 1923 in SEQ ID NO: 7.

-   [12] The method according to any of [9] to [11], further comprising     a step of determining whether an amplified nucleic acid fragment     with an intended size is obtained. -   [13] The method according to [12], further comprising a step of     determining that the subject is a subject positive for a fusion gene     of CLDN18 gene and ARHGAP6 gene or a subject positive for a fusion     gene of CLDN18 gene and ARHGAP26 gene when the amplified nucleic     acid fragment with the intended size is obtained, and determining     that the subject is not a subject positive for the fusion gene when     the fragment is not obtained. -   [14] The method according to any of [9] to [11], further comprising     a step of determining the nucleotide sequence of the amplified     nucleic acid fragment. -   [15] The method according to [14], further comprising a step of     determining the subject is a subject positive for a fusion gene of     CLDN18 gene and ARHGAP6 gene or a subject positive for a fusion gene     of CLDN18 gene and ARHGAP26 gene when the amplified nucleic acid     fragment comprises the nucleotide sequence of a part encoding CLDN18     and the nucleotide sequence of a part encoding ARHGAP6 or the     nucleotide sequence of a part encoding ARHGAP26 in the same     fragment, and determining that the subject is not a subject positive     for the fusion gene when the nucleotide sequences are not comprised     in the same fragment. -   [16] The method according to [8], comprising a step of hybridizing a     probe comprising an oligonucleotide that hybridizes with the     polynucleotide under stringent conditions with a nucleic acid in a     sample obtained from the subject. -   [17] The method according to [16], comprising a step of performing     in situ hybridization using a sample obtained from the subject, a     probe designed for a part encoding CLDN18 in the polynucleotide, and     a probe designed for a part encoding ARHGAP6 or a part encoding     ARHGAP26 in the polynucleotide. -   [18] The method according to [17], involving use of a plurality of     probes designed for a part encoding CLDN18 and a plurality of probes     designed for a part encoding ARHGAP6 or a part encoding ARHGAP26. -   [19] The method according to [17] or [18], involving

use of a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO: 1, and a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 751 to 3087 in SEQ ID NO: 1;

use of a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO: 3, and a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 751 to 2460 in SEQ ID NO: 3;

use of a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO: 5, and a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 751 to 2088 in SEQ ID NO: 5; or

use of a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO: 7, and a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 751 to 1923 in SEQ ID NO: 7,

in the step of hybridizing a probe with a nucleic acid in the sample obtained from the subject.

-   [20] The method according to any of [17] to [19], further comprising     a step of amplifying a signal of the hybridization. -   [21] The method according to any of [17] to [20], further comprising     a step of detecting an overlap of a signal from a probe designed for     a part encoding CLDN18 and a signal from a probe designed for a part     encoding ARHGAP6 or a part encoding ARHGAP26. -   [22] The method according to [21], further comprising a step of     determining that the subject is a subject positive for a fusion gene     of CLDN18 gene and ARHGAP6 gene or a subject positive for a fusion     gene of CLDN18 gene and ARHGAP26 gene when the presence of the two     signals at the same location is detected, and determining that the     subject is not a subject positive for the fusion gene when the     presence of the two signals at the same location is not detected. -   [23] The method according to any of [1] to [22], comprising a step     of obtaining a sample from the subject. -   [24] The method according to any of [1] to [23], wherein the sample     is body fluid or body cavity lavage fluid. -   [25] The method according to any of [1] to [24], wherein the sample     is ascites or peritoneal cavity lavage fluid.

Moreover, the present invention relates to the following [26] to [31].

-   [26] A method for identifying a subject suspected to have pancreatic     cancer or a subject having pancreatic cancer that is an indication     for a therapy with an ARHGAP6 inhibitor or an ARHGAP26 inhibitor     and/or an agent that blocks an abnormal signal induced by the fusion     gene of the CLDN18 gene and the ARHGAP6 gene or the fusion gene of     the CLDN18 gene and the ARHGAP26 gene, comprising a step according     to any of [1] to [22]. -   [27] The method according to [26], comprising a step of obtaining a     sample from the subject. -   [28] The method according to [26] or [27], further comprising a step     of determining that the subject is suitable for a therapy with an     ARHGAP6 inhibitor or an ARHGAP26 inhibitor and/or an agent that     blocks an abnormal signal induced by the fusion gene of the CLDN18     gene and the ARHGAP6 gene or the fusion gene of the CLDN18 gene and     the ARHGAP26 gene when a polynucleotide comprising the fusion point     of the CLDN18 gene and the ARHGAP6 gene or a polynucleotide     comprising the fusion point of the CLDN18 gene and the ARHGAP26 gene     is detected in a sample obtained from the subject, and determining     that the subject is not suitable for the therapy when the     polypeptide is not detected. -   [29] The method according to [26] or [27], further comprising the     step according to [12] and a step of determining that the subject is     suitable for a therapy with an ARHGAP6 inhibitor or an ARHGAP26     inhibitor and/or an agent that blocks an abnormal signal induced by     the fusion gene of the CLDN18 gene and the ARHGAP6 gene or the     fusion gene of the CLDN18 gene and the ARHGAP26 gene when an     amplified nucleic acid fragment with the intended size is obtained,     and determining that the subject is not suitable for the therapy     when the amplified nucleic acid fragment is not obtained. -   [30] The method according to [26] or [27], further comprising the     step according to [14] and a step of determining that the subject is     suitable for a therapy with an ARHGAP6 inhibitor or an ARHGAP26     inhibitor and/or an agent that blocks an abnormal signal induced by     the fusion gene of the CLDN18 gene and the ARHGAP6 gene or the     fusion gene of the CLDN18 gene and the ARHGAP26 gene when the     amplified nucleic acid fragment comprises the nucleotide sequence of     a part encoding CLDN18 and the nucleotide sequence of a part     encoding ARHGAP6 or the nucleotide sequence of a part encoding     ARHGAP26 in the same fragment, and determining that the subject is     not suitable for the therapy when the amplified nucleic acid     fragment does not comprise the nucleotide sequences. -   [31] The method according to [26] or [27], further comprising the     step according to [21] and a step of determining that the subject is     suitable for a therapy with an ARHGAP6 inhibitor or an ARHGAP26     inhibitor and/or an agent that blocks an abnormal signal induced by     the fusion gene of the CLDN18 gene and the ARHGAP6 gene or the     fusion gene of the CLDN18 gene and the ARHGAP26 gene when the     presence of the two signals at the same location is detected, and     determining that the subject is not suitable for the therapy when     the presence of the two signals at the same location is not     detected.

Moreover, the present invention relates to the following [32] to [34].

-   [32] A primer set for detecting the fusion gene of the CLDN18 gene     and the ARHGAP6 gene or the fusion gene of the CLDN18 gene and the     ARHGAP26 gene in a sample from a subject suspected to have     pancreatic cancer or a subject having pancreatic cancer, comprising     a sense primer designed for a part encoding CLDN18 and an antisense     primer designed for a part encoding ARHGAP6 or a part encoding     ARHGAP26, wherein the antisense primer consists of an     oligonucleotide that hybridizes with the polynucleotide according to     any of [1] to [6] under stringent conditions and the sense primer     consists of an oligonucleotide that hybridizes with a complementary     strand to the polynucleotide under stringent conditions. -   [33] The primer set according to [32], wherein

the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 1 under stringent conditions, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide consisting of nucleotides at positions 751 to 3087 in SEQ ID NO: 1 under stringent conditions;

the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 3 under stringent conditions, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide consisting of nucleotides at positions 751 to 2460 in SEQ ID NO: 3 under stringent conditions;

the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 5 under stringent conditions, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide consisting of nucleotides at positions 751 to 2088 in SEQ ID NO: 5 under stringent conditions; or

the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 7 under stringent conditions, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide consisting of nucleotides at positions 751 to 1923 in SEQ ID NO: 7 under stringent conditions.

-   [34] The primer set according to [32] or [33], wherein

the sense primer consists of an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 1 to 750 in SEQ ID NO: 1, and the antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 751 to 3087 in SEQ ID NO: 1;

the sense primer consists of an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 1 to 750 in SEQ ID NO: 3 and the antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 751 to 2460 in SEQ ID NO: 3;

the sense primer consists of an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 1 to 750 in SEQ ID NO: 5, and the antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 751 to 2088 in SEQ ID NO: 5; or

the sense primer consists of an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 1 to 750 in SEQ ID NO: 7, and the antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases between nucleotides at positions 751 to 1923 in SEQ ID NO: 7.

Moreover, the present invention relates to the following [35] to [38].

-   [35] A probe for detecting a fusion gene of CLDN18 gene and ARHGAP6     gene or a fusion gene of CLDN18 gene and ARHGAP26 gene in a sample     from a subject suspected to have pancreatic cancer or a subject     having pancreatic cancer, comprising an oligonucleotide that     hybridizes with the polynucleotide according to any of [1] to [6]     under stringent conditions. -   [36] A probe set comprising a plurality of probes according to [35],     comprising a probe designed for a part encoding CLDN18 and a probe     designed for a part encoding ARHGAP6 or a part encoding ARHGAP26 in     the polynucleotide according to any of [1] to [6]. -   [37] The probe set according to [36], comprising a plurality of     probes designed for a part encoding CLDN18 and a plurality of probes     designed for a part encoding ARHGAP6 or a part encoding ARHGAP26. -   [38] A probe set of [36] or [37], comprising

a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO: 1, and a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 751 to 3087 in SEQ ID NO: 1

a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO: 3, and a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 751 to 2460 in SEQ ID NO: 3;

a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO: 5, and a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 751 to 2088 in SEQ ID NO: 5; or

a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO: 7, and a plurality of flanking probe pairs comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases of nucleotides at positions 751 to 1923 in SEQ ID NO: 7.

Moreover, the present invention relates to following [39] to [41].

-   [39] A kit for detecting a fusion gene of CLDN18 gene and ARHGAP6     gene or a fusion gene of CLDN18 gene and ARHGAP26 gene in a sample     from a subject suspected to have pancreatic cancer or a subject     having pancreatic cancer, comprising the primer set according to any     of [32] to [34], -   [40] A kit for detecting a fusion gene of CLDN18 gene and ARHGAP6     gene or a fusion gene of CLDN18 gene and ARHGAP26 gene in a sample     obtained from a subject suspected to have pancreatic cancer or a     subject having pancreatic cancer, comprising a probe or the probe     set according to any of [35] to [38], -   [41] A kit for detection according to [40], further comprising a     reagent for amplifying a signal of the hybridization.

Moreover, the present invention relates to the following [42] to [47].

-   [42] A method for selecting a subject positive for fusion protein of     CLDN18 and ARHGAP6 or a subject positive for a fusion protein of     CLDN18 and ARHGAP26, comprising a step of detecting the presence of     a polypeptide of the following (1) or (2) in a sample obtained from     a subject suspected to have pancreatic cancer or a subject having     pancreatic cancer: -   (1) a polypeptide comprising an amino acid sequence having 90% or     more identity with an amino acid sequence set forth in SEQ ID NO: 2,     4, 6, or 8; -   (2) a polypeptide having an amino acid sequence modified from an     amino acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8 by     deletion, substitution, insertion, and/or addition of 1 to 10 amino     acids. -   [43] The method according to [42], wherein the polypeptide has tumor     promoting ability. -   [44] The method according to [42], wherein the polypeptide is a     polypeptide consisting of an amino acid sequence set forth in SEQ ID     NO: 2, 4, 6, or 8. -   [45] The method according to any of [42] to [44], wherein the step     of detecting the presence of the polypeptide comprises a step of     bringing an antibody (primary antibody) that recognizes a part     derived from the CLDN18 gene in the polypeptide and an antibody     (primary antibody) that recognizes a part derived from the ARHGAP6     gene or a part derived from the ARHGAP26 gene in the polypeptide in     contact with the sample obtained from the subject. -   [46] The method according to [45], further comprising the steps     of i) to v) as follows:

i) a step of adding oligonucleotide-conjugated secondary antibodies that respectively bind to the primary antibodies; ii) a step of adding a ligation solution containing 2 oligonucleotides partially complementary to the oligonucleotides conjugated to the secondary antibodies and a ligase capable of ligating the 2 oligonucleotides when they come in the vicinity to form a ring structure and making ligation to cause a ligation reaction; iii) a step of extending a nucleic acid along the formed ring structure; iv) a step of hybridizing a labelled oligonucleotide probe capable of hybridizing the extended nucleic acid; and v) a step of detecting the label signal.

-   [47] The method according to any of [42] to [46], comprising a step     of obtaining a sample from the subject.

Moreover, the present invention relates to the following [48] to [50].

-   [48] A method for identifying a subject suspected to have pancreatic     cancer or a subject having pancreatic cancer that is an indication     for a therapy with an ARHGAP6 inhibitor or an ARHGAP26 inhibitor     and/or an agent that blocks an abnormal signal induced by the fusion     gene of the CLDN18 gene and the ARHGAP6 gene or the fusion gene of     the CLDN18 gene and the ARHGAP26 gene, comprising a step according     to any of [42] to [46]. -   [49] The method according to [48], comprising a step of obtaining a     sample from the subject. -   [50] The method according to [48] or [49], further comprising a step     of determining that the subject is suitable for a therapy with an     ARHGAP6 inhibitor or an ARHGAP26 inhibitor and/or an agent that     blocks an abnormal signal induced by the fusion gene of the CLDN18     gene and the ARHGAP6 gene or the fusion gene of the CLDN18 gene and     the ARHGAP26 gene when the polypeptide according to any of [42] to     [44] is detected in a sample obtained from the subject, and     determining that the subject is not suitable for the therapy when     the polypeptide is not detected.

Moreover, the present invention relates to the following [51] to [52].

-   [51] A kit for detecting a fusion protein of CLDN18 and ARHGAP6 or a     fusion protein of CLDN18 and ARHGAP26 in a sample obtained from a     subject suspected to have pancreatic cancer or a subject having     pancreatic cancer, comprising an antibody (primary antibody) that     recognizes a part derived from the CLDN18 gene in the polypeptide     according to any of [42] to [44] and an antibody (primary antibody)     that recognizes a part derived from the ARHGAP6 gene or a part     derived from the ARHGAP26 gene in the polypeptide. -   [52] A kit for detection according to [51], comprising     oligonucleotide-conjugated secondary antibodies that respectively     bind to the primary antibodies, 2 oligonucleotides partially     complementary to the oligonucleotides conjugated to the secondary     antibodies, a ligase capable of ligating the 2 oligonucleotides when     they are in the vicinity to form a ring structure, a polymerase     capable of extending a nucleic acid along the ring structure, and a     labelled oligonucleotide probe.

Moreover, the present invention relates to the following [53].

-   [53] A polypeptide according to the following (1) or (2) or a     polynucleotide encoding the polypeptide: -   (1) a polypeptide comprising an amino acid sequence having 90% or     more identity with an amino acid sequence set forth in SEQ ID NO: 2,     4, 6, or 8; -   (2) a polypeptide having an amino acid sequence modified from an     amino acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8 by     deletion, substitution, insertion, and/or addition of 1 to 10 amino     acids.

Moreover, the present invention relates to the following [54].

-   [54] A polypeptide according to any of the following (1) to (3) or a     polynucleotide encoding the polypeptide: -   (1) a polypeptide comprising an amino acid sequence having 90% or     more identity with the amino acid sequence set forth in SEQ ID NO:     2, 4, 6, or 8 and having tumor promoting ability; -   (2) a polypeptide comprising the amino acid sequence set forth in     SEQ ID NO: 2, 4, 6, or 8 and having tumor promoting ability or a     polypeptide having an amino acids sequence modified from the amino     acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8 by deletion,     substitution, insertion, and/or addition of 1 to 10 amino acids and     having tumor promoting ability; -   (3) a polypeptide consisting of the amino acid sequence set forth in     SEQ ID NO: 2, 4, 6, or 8.

Advantageous Effects of Invention

The method according to the present invention is expected to be useful to detect pancreatic cancer positive for a fusion gene of CLDN18 gene and ARHGAP6 gene or a fusion gene of CLDN18 gene and ARHGAP26 gene (hereinafter, also referred to as “CLDN18-ARHGAP6 fusion gene”, “CLDN18-ARHGAP26 fusion gene”, “CLDN18-ARHGAP6/26 fusion gene”, or “fusion gene of the present invention”), or to identify subjects for therapies with agents that blocks signals induced by a fusion gene indicated in the present invention. The primer sets, probes, probe sets, and kits for detection of the present invention can be used in the method according to the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the result of electrophoresis of PCR products which were amplified the region containing a fusion point of CLDN18-ARHGAP6 fusion gene by PCR from cDNA, as a template, prepared from total RNA purified from ascites of pancreatic cancer patients.

FIG. 2 shows the result of electrophoresis of PCR products which were amplified the region containing a fusion point of CLDN18-ARHGAP26 fusion gene by PCR from cDNA, as a template, prepared from total RNA purified from ascites of pancreatic cancer patients.

FIG. 3 shows the result of nucleotide sequencing of the regions containing the fusion point of CLDN18-ARHGAP6/26 fusion gene product amplified by PCR. The underlined region indicates a sequence corresponding to ARHGAP6 or ARHGAP26.

FIG. 4 shows the result of electrophoresis of PCR products which were amplified the region containing a fusion point of CLDN18-ARHGAP26 fusion gene by PCR from cDNA, as a template, prepared from a gastric cancer cell line NSC-47C.

FIG. 5 shows the result of western blotting, confirming that the expression of CLDN18-ARHGAP26 fusion proteins were suppressed in a CLDN18 siRNA group and an ARHGAP26 siRNA (Wild & Fusion) group.

FIG. 6 shows the result of cell viability assay confirming that the viability of a gastric cancer cell line NSC-47C were attenuated significantly by treating the gastric cancer cell line with an siRNA targeting CLDN18-ARHGAP26 fusion gene.

DESCRIPTION OF EMBODIMENTS

<Method according to the Present Invention>

The method according to the present invention includes 1. a method for selecting a subject positive for a fusion gene, and 2. a method for selecting a subject positive for a fusion protein encoded by a fusion gene (collectively hereinafter also referred to as the “selection method according to the present invention”). Furthermore, the method according to the present invention includes 3. a method for identifying a subject for which a therapy with an ARHGAP6 inhibitor or an ARHGAP26 inhibitor and/or an agent that blocks an abnormal signal induced by a fusion gene of CLDN18 gene and ARHGAP6 gene or a fusion gene of the CLDN18 gene and ARHGAP26 gene is suitable (hereinafter also referred to as the “identification method according to the present invention”). The method according to the present invention includes a step of detecting the presence of a particular polynucleotide or polypeptide in a sample obtained from a subject.

In the method according to the present invention, a sample obtained from a subject may be a collected material from a subject (a sample separated from a living body), such as any collected cell, tissue, body fluid, and body cavity lavage fluid. It is preferable to use ascites which is body fluid, and peritoneal lavage fluid which is body cavity lavage fluid. It is particularly preferable to use ascites. As the body cavity lavage fluid, it may be used lavage fluid which is obtained by introducing lavage fluid (e.g. saline) into a subject's body cavity (peritoneal or the like) during laparotomy or by catheterization, performing a lavage of the body cavity, and then collecting the lavage fluid. It is known that such lavage fluid is provided for cytology. From the sample obtained from a subject, genomic DNA may be extracted to be used. Alternatively, a transcription product thereof (a resulting product of the transcription or translation of the genomic DNA; e.g., RNA or protein) or cDNA prepared from the RNA may be used. In particular, it is preferable to prepare and use RNA or cDNA. The sample may be used after subjecting a sample collected from a subject into a pretreatment such as dilution or concentration, a precipitation treatment by centrifugation, or an addition of a blood coagulation inhibitor such as heparin, as necessary. Alternatively, the sample may be used as it is without such a pretreatment. Further, the sample may be used as a stabilized specimen wherein a sample is formalin-fixed and embedded in paraffin (Formalin-Fixed Paraffin-Embedded sample; FFPE specimen). It may be used as a thinly sliced FFPE section of the FFPE specimen. With the FFPE section, it is possible to directly detect a polynucleotide or polypeptide present therein. In one embodiment, the method according to the present invention includes a step of obtaining a sample from a subject.

In the method according to the present invention, a “fusion gene of CLDN18 gene and ARHGAP6 gene” or a “fusion gene of CLDN18 gene and ARHGAP26 gene” refers to a fusion gene containing a part of a CLDN18 gene and a part of an ARHGAP6 gene, or a fusion gene containing a part of CLDN18 gene and a part of ARHGAP26 gene. Representative examples of the fusion gene of the CLDN18 gene and the ARHGAP6 gene include a polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 1 or a polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 3. Representative examples of the fusion gene of the CLDN18 gene and the ARHGAP26 gene include a polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 5 or a polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 7.

The polynucleotide consisting of the nucleotide sequence set forth in SEQ ID NO: 1 is a polynucleotide having a nucleotide sequence of nucleotides at positions 54 (corresponding to 5′ end of the coding sequence (hereinafter referred to as CDS)) to 803 in CLDN18 gene (NCBI accession number: NM_001002026.2) and nucleotides at positions 1462 to 3798 in ARHGAP6 gene (NCBI accession number: NM_013427.2). The polynucleotide consisting of the nucleotide sequence set forth in SEQ ID NO: 3 is a polynucleotide having a nucleotide sequence of nucleotides at positions 54(corresponding to 5′ end of CDS) to 803 in CLDN18 gene (NCBI accession number: NM_001002026.2) and nucleotides at positions 1462 to 3171 in ARHGAP6 gene (NCBI accession number: NM_006125.2).

The polynucleotide consisting of the nucleotide sequence set forth in SEQ ID NO: 5 is a polynucleotide having a nucleotide sequence of nucleotides at positions 54 (corresponding to 5′ end of CDS) to 803 in CLDN18 gene (NCBI accession number: NM_001002026.2) and nucleotides at positions 1143 to 2480 in ARHGAP26 gene (NCBI accession number: NM_015071.4)

The polynucleotide consisting of the nucleotide sequence set forth in SEQ ID NO: 7 is a polynucleotide having a nucleotide sequence of nucleotides at positions 54 (corresponding to 5′ end of CDS) to 803 in CLDN18 gene (NCBI accession number: NM_001002026.2) and nucleotides at positions 1143 to 2315 in ARHGAP26 gene (NCBI accession number: NM_001135608.1).

In the nucleotide sequence set forth in SEQ ID NO: 1, a sequence of nucleotides at positions 1 to 750 is derived from the CLDN18 gene, and a sequence of nucleotides at positions 751 to 3087 is derived from the ARHGAP6 gene and, in the nucleotide sequence set forth in SEQ ID NO: 3, a sequence of nucleotides at positions 1 to 750 is derived from the CLDN18 gene, and a sequence of nucleotides at positions 751 to 2460 is derived from the ARHGAP6 gene. Meanwhile, in the nucleotide sequence set forth in SEQ ID NO: 5, a sequence of nucleotides at positions 1 to 750 is derived from the CLDN18 gene, and a sequence of nucleotides at positions 751 to 2088 is derived from the ARHGAP26 gene, and in the nucleotide sequence set forth in SEQ ID NO: 7, a sequence of nucleotides at positions 1 to 750 is derived from the CLDN18 gene, and a sequence of nucleotides at positions 751 to 1923 is derived from the ARHGAP26 gene.

The polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 1, 3, 5 or 7 is also referred to as a “fusion polynucleotide”. The amino acid sequence encoded by nucleotides at positions 1 to 3087 in SEQ ID NO: 1 (including a stop codon) is set forth in SEQ ID NO: 2, and the amino acid sequence encoded by nucleotides at positions 1 to 2460 in SEQ ID NO: 3 (including a stop codon) is set forth in SEQ ID NO: 4. The amino acid sequence encoded by nucleotides at positions 1 to 2088 in SEQ ID NO: 5 (including a stop codon) is set forth in SEQ ID NO: 6, and the amino acid sequence encoded by nucleotides at positions 1 to 1923 in SEQ ID NO: 7 (including the stop codon) is set forth in SEQ ID NO: 8.

1. Method for Selecting a Subject Positive for a Fusion Gene

In the “step of detecting the presence of a polynucleotide” in the method for selecting a subject positive for a fusion gene according to the present invention (herein referred to as the “step of detecting the presence of a polynucleotide” according to the present invention), the polynucleotide to be used as the detection target (herein referred to as the “detection target polynucleotide”) may be a polynucleotide containing a fusion point of CLDN18 gene and ARHGAP6 gene or a polynucleotide containing a fusion point of CLDN18 gene and ARHGAP26 gene. The “fusion point” in the detection target polynucleotide means a point in which a part derived from CLDN18 gene and a part derived from ARHGAP6 gene in the detection target polynucleotide are fused or in which a part derived from CLDN18 gene and a part derived from ARHGAP26 gene in the detection target polynucleotide are fused.

In one embodiment, the detection target polynucleotide may be a polynucleotide encoding a polypeptide of (1) or (2) as follows:

-   (1) a polypeptide comprising an amino acid sequence having 90% or     more identity with an amino acid sequence set forth in SEQ ID NO: 2,     4, 6, or 8; -   (2) a polypeptide having an amino acid sequence modified from an     amino acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8 by     deletion, substitution, insertion, and/or addition of 1 to 10 amino     acids.

The polypeptide may further have tumor promoting ability.

In the polypeptide, the “identity with an amino acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8” is preferably 95% or more, more preferably 98% or more.

It should be noted that the “identity” used herein with reference to an amino acid sequence means a value Identity obtained by using parameters described below in a NEEDLE program (J Mol Biol. 1970; 48(3):443-453). The parameters are as follows:

Gap penalty=10

Extend penalty=0.5

Matrix=BLOSUM62

In the polypeptide, the amino acid sequence is modified from an amino acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8 by deletion, substitution, insertion, and/or addition of 1 to several amino acids, preferably 1 to 10 amino acids, more preferably 1 to 7 amino acids, further preferably 1 to 5 amino acids.

Examples of the method for confirming that a polypeptide “has a tumor promoting ability” include a method having introducing an siRNA targeting a polynucleotide encoding the polypeptide into a cell capable of expressing the polypeptide, and then confirming that the viability of the cell decreases, as is exemplified by the method described in Reference Example 2.

In one embodiment, the detection target polynucleotide is a polynucleotide encoding a polypeptide of (1) or (2) as follows:

-   (1) a polypeptide having an amino acid sequence modified from an     amino acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8 by     deletion, substitution, insertion, and/or addition of 1 to 10 amino     acids and having tumor promoting ability; -   (2) a polypeptide having an amino acid sequence set forth in SEQ ID     NO: 2, 4, 6, or 8 and having tumor promoting ability.

In one embodiment, the detection target polynucleotide is a polynucleotide encoding a polypeptide consisting of an amino acid sequence set forth in SEQ ID NO: 2, 4, 6, or 8.

Examples of the polynucleotide encoding a “polypeptide consisting of an amino acid sequence set forth in SEQ ID NO: 2” include a “polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 1”. Examples of the polynucleotide encoding a “polypeptide consisting of an amino acid sequence set forth in SEQ ID NO: 4” include a “polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 3”. Examples of the polynucleotide encoding a “polypeptide consisting of an amino acid sequence set forth in SEQ ID NO: 6” include a “polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 5”. Examples of the polynucleotide encoding a “polypeptide consisting of an amino acid sequence set forth in SEQ ID NO: 8” include a “polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 7”. In one embodiment, the detection target polynucleotide is a polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 1, 3, 5 or 7.

Of the nucleotide sequence set forth in SEQ ID NO: 5 or 7, a sequence of nucleotides at positions 711 to 790 (a sequence having each 40 nucleotides of before and after the fusion point, a total of 80 nucleotides) is set forth in SEQ ID NO: 17. Of the nucleotide sequence set forth in SEQ ID NO: 1 or 3, a sequence of nucleotides at positions 711 to 790 (a sequence having each 40 nucleotides of before and after the fusion point, a total of 80 nucleotides) is set forth in SEQ ID NO: 19. The sequence modified from a sequence set forth in SEQ ID NO: 19 by single nucleotide substitution is a sequence set forth in SEQ ID NO: 18, 20, or 21.

In one embodiment, the detection target polynucleotide may be a polynucleotide of (3) or (4) as follows:

-   (3) a polynucleotide comprising a nucleotide sequence having 90% or     more identity with a nucleotide sequence set forth in SEQ ID NO: 17     or 19; -   (4) a polynucleotide comprising a nucleotide sequence modified from     a nucleotide sequence set forth in SEQ ID NO: 17 or 19 by deletion,     substitution, insertion, and/or addition of 1 to 10 nucleotides.

In the polynucleotide, the “identity with a nucleotide sequence set forth in SEQ ID NO: 17 or 19” is preferably 95% or more, and more preferably 98% or more.

It should be noted that the “identity” used herein with reference to a nucleotide sequence means a value Identity obtained by using parameters described below in a NEEDLE program (J Mol Biol. 1970; 48(3):443-453).

Gap penalty=10

Extend penalty=0.5

Matrix=DNA full

In the polynucleotide, the nucleotide sequence is modified from a nucleotide sequence set forth in SEQ ID NO: 17 or 19 by deletion, substitution, insertion, and/or addition of 1 to several nucleotides, preferably 1 to 10 nucleotides, more preferably 1 to 7 nucleotides, further preferably 1 to 5 nucleotides, still preferably 1 to 3 nucleotides, still more preferably 1 or 2 nucleotides. Examples of the sequence modified from a nucleotide sequence set forth in SEQ ID NO: 19 by single nucleotide substitution include a sequence set forth in SEQ ID NO: 18, 20, or 21.

In one embodiment, the detection target polynucleotide is a polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 17, 18, 19, 20, or 21.

The method for selecting a subject positive for a fusion gene according to the present invention may include either identifying a subject positive for a fusion gene of CLDN18 gene and ARHGAP6 gene or a fusion gene of CLDN18 gene and ARHGAP26 gene or identifying a subject negative for a fusion gene of a CLDN18 gene and ARHGAP6 gene or a fusion gene of CLDN18 gene and ARHGAP26 gene, or both.

In one embodiment, the method for selecting a subject positive for a fusion gene according to the present invention may further include a step of determining that the subject is a subject positive for a fusion gene of CLDN18 gene and ARHGAP6 gene or a subject positive for a fusion gene of CLDN18 gene and ARHGAP26 gene when the presence of the detection target polynucleotide is detected and determining that the subject is not a subject positive for the fusion gene when the presence of the detection target polynucleotide is not detected.

The method for selecting a subject positive for a fusion gene according to the present invention may include a step of amplifying a nucleic acid in a sample obtained from the subject (additional step A) and/or hybridizing a probe with a nucleic acid in a sample obtained from the subject (additional step B), for detecting the presence of detection target the polynucleotide.

The nucleic acid used may be a genomic DNA, RNA, or cDNA prepared from RNA. The methods for extraction of genomic DNA, extraction of RNA, and preparation of cDNA from RNA are known in the art, and they can be conveniently performed using a commercially available DNA extraction kit, RNA extraction kit, or cDNA synthesis kit.

The step of amplifying a nucleic acid in a sample obtained from the subject can be performed using a known nucleic acid amplification method. Examples of such a nucleic acid amplification method include PCR (Polymerase chain reaction, for example, real-time quantitative PCR), LCR (Ligase chain reaction), SDA (Strand displacement amplification), NASBA (Nucleic acid sequence-based amplification), ICAN (Isothermal and chimeric primer-initiated amplification of nucleic acids), LAMP (Loop-mediated isothermal amplification), and TMA (Transcription-mediated amplification). Preferred examples thereof include PCR.

Specifically, a nucleic acid in a sample obtained from a subject (for example, genomic DNA, RNA, or cDNA prepared from RNA) is subjected to a nucleic acid amplification reaction using a primer set designed to be capable of specifically amplifying the detection target polynucleotide, to be amplified. The primer set used is not particularly limited as long as it is capable of specifically amplifying the detection target polynucleotide. For example, such a primer set can be easily designed by those skilled in the art based on a nucleotide sequence of the detection target polynucleotide using a primer design software (for example, Primer Express®; Thermo Fisher Scientific K.K.) or the like. More specifically, the primer set contains a sense primer (5′-primer) designed for a part encoding CLDN18 in the detection target polynucleotide (e.g., any part within CLDN18 gene region of the fusion polynucleotide (particularly cDNA)) and an antisense primer (3′-primer) designed for a part encoding ARHGAP6 or a part encoding ARHGAP26 in the detection target polynucleotide (e.g., any part within ARHGAP6 or ARHGAP26 gene region of the fusion polynucleotide (particularly, cDNA)), wherein the antisense primer consists of an oligonucleotide that hybridizes with the detection target polynucleotide under stringent conditions (preferably, highly stringent conditions) and the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the detection target polynucleotide under stringent conditions (preferably, highly stringent conditions). Alternatively, one of the sense primer or antisense primer may be designed so as to correspond to a region containing a fusion point in the detection target polynucleotide.

The “stringent condition” used herein refers to a condition for hybridization of “5×SSPE, 5× Denhardt's solution, 0.5% SDS, 50% formamide, 200 μg/mL salmon sperm DNA, overnight at 42° C.”, and a condition for washing of “0.5× SSC, 0.1% SDS, 42° C.”. The “highly stringent condition” refers to a condition for hybridization of “5×SSPE, 5× Denhardt's solution, 0.5% SDS, 50% formamide, 200 μg/mL salmon sperm DNA, overnight at 42° C.”, and a condition for washing of “0.2×SSC, 0.1% SDS, 65° C”.

As used herein, the “region containing a fusion point” in a detection target polynucleotide refers to, for example, a region containing nucleotides of nucleotides at positions 750 and 751 when the detection target polynucleotide is a polynucleotide consisting of a nucleotide sequence set forth in SEQ ID NO: 1, 3, 5 or 7, respectively. For example, the “region containing a fusion point” in the detection target polynucleotide refers to a region containing nucleotides of nucleotides at positions 40 and 41 when the detection target polynucleotide is a polynucleotide consisting of the nucleotide sequence set forth in SEQ ID NO: 17, 18, 19, 20 or 21, respectively.

In one embodiment, as a primer set, it is possible to use a primer set according to the present invention described below.

In a preferred embodiment, the method for selecting a subject positive for a fusion gene according to the present invention may further include, in addition to a step of amplifying a nucleic acid in a sample obtained from the subject, a step of determining whether an amplified nucleic acid fragment with an intended size is obtained (additional step C). The step of determining whether an amplified nucleic acid fragment with an intended size is obtained may be performed, for example, using electrophoresis. In the electrophoresis, for example, a nucleic acid fragment may be analyzed by agarose gel electrophoresis and stained by ethidium bromide or the like to confirm whether an amplified nucleic acid fragment with an intended size is obtained.

Further, by performing PCR amplification monitoring during the gene amplification process (real-time quantitative PCR) (Genome Res. 1996; 6(10):986-994), it is possible to make a more quantitative analysis for the amplified nucleic acid fragment. In the PCR amplification monitoring, for example, ABI PRISM®7900 (Thermo Fisher Scientific K.K.) may be used.

When the amplified nucleic acid fragment with an intended size is obtained, it is considered that the detection target polynucleotide is present in a sample obtained from a subject. In one embodiment, the method for selecting a subject positive for a fusion gene according to the present invention may further include a step of determining that the subject is a subject positive for a fusion gene of CLDN18 gene and ARHGAP6 gene or a subject positive for a fusion gene of CLDN18 gene and ARHGAP26 gene when the amplified nucleic acid fragment with the intended size is obtained and determining that the subject is not a subject positive for the fusion gene when the fragment with the intended size is not obtained.

In another preferred embodiment, the method for selecting a subject positive for a fusion gene according to the present invention may further include, in addition to a step of amplifying a nucleic acid in a sample obtained from the subject, a step of determining the nucleotide sequence of the amplified nucleic acid fragment (additional step D). The step of determining the nucleotide sequence of the amplified nucleic acid fragment may be performed, for example, using a method known in the art such as a Sanger sequencing method (for example, ABI PRISM® 3100 (Thermo Fisher Scientific K.K.) may be used), and next-generation sequencing methods including a sequence by synthesis method (Nat Biotechnol. 2008; 26 (10): 1135-1145) (for example, HiSeq2500 (Illumina, Inc.) may be used).

The step of determining a nucleotide sequence of a nucleic acid fragment include not only a step of determining a sequence of the full length of the nucleic acid fragment, but also a step of determining a partial sequence containing both ends or a partial sequence containing a fusion point of the nucleic acid fragment.

When the sequenced nucleic acid fragment contains a nucleotide sequence of a part encoding CLDN18 and a nucleotide sequence of a part encoding ARHGAP6 or a nucleotide sequence of a part encoding ARHGAP26 in a detection target polynucleotide in the same fragment, it is considered that the detection target polynucleotide is present in the sample obtained from a subject. In one embodiment, the method for selecting a subject positive for a fusion gene according to the present invention may further include a step of determining the subject is a subject positive for a fusion gene of CLDN18 gene and ARHGAP6 gene or a subject positive for a fusion gene of CLDN18 gene and ARHGAP26 gene when the amplified nucleic acid fragment comprises the nucleotide sequence of a part encoding CLDN18 and the nucleotide sequence of a part encoding ARHGAP6 or the nucleotide sequence of a part encoding ARHGAP26 in the detection target polynucleotide in the same fragment and determining that the subject is not a subject positive for the fusion gene when the nucleotide sequences are not comprised in the same fragment.

The step of hybridizing a probe with a nucleic acid in a sample obtained from the subject may be performed using a known hybridization method using a probe comprising an oligonucleotide that hybridizes with the detection target polynucleotide under stringent conditions (preferably, highly stringent conditions). Examples of such a method include northern hybridization, a dot blot method, a DNA microarray method, an RNA protection method, and in situ hybridization. Preferred methods thereof include in situ hybridization. Detection using in situ hybridization techniques may be performed, for example, by a known method such as a fluorescence in situ hybridization (FISH) method, a chromogenic in situ hybridization (CISH) method, or a silver in situ hybridization (SISH) method. The chain length of a probe used in the hybridization may be appropriately selected by those skilled in the art depending on a hybridization method to be used, but the probe preferably has a chain length of at least 16 bases. In one embodiment, as a probe, it is possible to use a probe according to the invention described below.

In one embodiment, the step of hybridizing a probe with a nucleic acid in a sample obtained from the subject can be performed in accordance with a known RNA in situ hybridization (RNA ISH) method (J Mol Diagn. 2012; 14(1):22-29). More specifically, in situ hybridization is performed using a sample obtained from a subject (for example, ascites, peritoneal cavity lavage fluid, or the like), a probe designed for a part encoding CLDN18 in the detection target polynucleotide (e.g., any part within CLDN18 gene region of the fusion polynucleotide), and a probe designed for a part encoding ARHGAP6 or a part encoding ARHGAP26 in the detection target polynucleotide (e.g., any point within ARHGAP6 or ARHGAP26 gene region of the fusion polynucleotide). Each of the probe contains an oligonucleotide that hybridizes with the detection target polynucleotide under stringent conditions (preferably, highly stringent conditions).

In one embodiment, in situ hybridization is performed using a plurality of detection probes designed for a part encoding CLDN18 and a plurality of detection probes designed for a part encoding ARHGAP6 or a part encoding ARHGAP26.

In one embodiment, in situ hybridization is performed using a probe according to the present invention described below.

In a further embodiment, a probe pair used in situ hybridization may include a flanking probe pair comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases in a 5′-untranslated region of CLDN18 gene (a region consisting of nucleotides at positions 1 to 53 in NCBI accession number: NM_001002026.2) and/or a flanking probe pair comprising an oligonucleotide complementary to an oligonucleotide of at least any 16 consecutive bases in a 3′-untranslated region of ARHGAP6 gene (a region consisting of nucleotides at positions 3799 to 5118 in NCBI accession number: NM_013427.2 or nucleotides at positions 3172 to 3632 in NCBI accession number: NM_006125.2) or in a 3′-untranslated region of ARHGAP26 gene (a region consisting of nucleotides at positions 2481 to 9041 in NCBI accession number: NM_015071.4 or nucleotides at positions 2316 to 8876 in NCBI accession number: NM_001135608.1).

As used herein, a “flanking probe pair” is composed of two probes hybridizing with the detection target polynucleotide side by side. Each of the probes contains an oligonucleotide complementary to the detection target polynucleotide, and the length of the oligonucleotide is typically at least 16 bases, preferably at least 18 bases. In one embodiment, the length of the oligonucleotide is 16 to 30 bases, preferably 18 to 25 bases.

In a preferred embodiment, the method for selecting a subject positive for a fusion gene according to the present invention may include, in addition to a step of performing in situ hybridization using a sample obtained from the subject, a step of amplifying a signal of the hybridization (additional step E). The step of amplifying a signal of the hybridization may be performed, for example, by hybridizing a reagent for amplifying the signal of the hybridization with a probe hybridized with a nucleic acid in a sample.

Examples of the reagent for amplifying the signal of the hybridization used during in situ hybridization include PreAmplifier Mix QT, Amplifier Mix QT, Label Probe Mix, and Label Probe Diluent QF, which are available from Affymetrix.

In a more preferred embodiment, the method for selecting a subject positive for a fusion gene according to the present invention further include a step of detecting an overlap of a signal from a probe designed for a part encoding CLDN18 and a signal from a probe designed for a part encoding ARHGAP6 or a part encoding ARHGAP26 (additional step F). By using different fluorescent or chromogenic reagents for detecting each of a probe designed for a part encoding CLDN18 and a probe designed for a part encoding ARHGAP6 or a part encoding ARHGAP26, it is possible to observe whether the signals from the two different probes are present at the same location (within the same molecule). When the presence of the two signals at the same location (within the same molecule) is detected, it is considered that the detection target polynucleotide is present in a sample obtained from the subject. In one embodiment, the method for selecting a subject positive for a fusion gene according to the present invention may further include a step of determining that the subject is a subject positive for a fusion gene of CLDN18 gene and ARHGAP6 gene or a subject positive for a fusion gene of CLDN18 gene and ARHGAP26 gene when the presence of the two signals at the same location (within the same molecule) is detected and determining that the subject is not a subject positive for the fusion gene when the presence of the two signals at the same location is not detected.

Each probe is not particularly limited. For example, the probe may be produced by a chemical synthesis method.

2. Method for Selecting Subject Positive for Fusion Protein Encoded by Fusion Gene

The method for selecting a subject positive for a fusion protein according to the present invention is a method for detecting a subject positive for a “fusion protein of CLDN18 and ARHGAP6 or fusion protein of CLDN18 and ARHGAP26” (herein, referred to as the “fusion protein of the present invention”). The fusion protein is a fusion protein encoded by a fusion gene of CLDN18 gene and ARHGAP6 gene or a fusion protein encoded by a fusion gene of CLDN18 gene and ARHGAP26 gene.

Examples of the polypeptide to be detected in the “step of detecting the presence of a polypeptide” in the method for selecting a subject positive for a fusion protein according to the present invention (herein, referred to as the “step of detecting the presence of a polypeptide”) (herein, referred to as the “polypeptide to be detected”) include polypeptides encoded by polynucleotides to be detected.

The method for selecting a subject positive for a fusion protein according to the present invention may comprises either of identifying a subject positive for a fusion protein of CLDN18 and ARHGAP6 or a subject positive for a fusion protein of CLDN18 and ARHGAP26 and identifying a subject negative for a fusion protein of CLDN18 and ARHGAP6 or a subject negative for a fusion protein of CLDN18 and ARHGAP26.

The method for selecting a subject positive for a fusion protein according to the present invention may further comprises a step of determining that a subject is a subject positive for a fusion protein of CLDN18 and ARHGAP6 or a subject positive for a fusion protein of CLDN18 and ARHGAP26 when a polypeptide to be detected is detected and determining that a subject is not a subject positive for the fusion protein when the polypeptide is not detected.

For example, the step of detecting the presence of a polypeptide can be conducted by an immunological assay or an enzymatic activity assay or a combined method thereof for detection by preparing a lysate derived from a sample (for example, cancer tissue, cancer cells or body fluid such as ascites obtained from the subject) obtained from the subject and combining the polypeptide to be detected contained in the lysate with antibodies to the proteins composing the fusion protein or mass spectrometry. Moreover, this step may be conducted by a method for detection by an immunohistological staining technique by the combination of the polypeptide to be detected contained in a sample (for example, an FFPE section or a precipitate) obtained from the subject pretreated (for example, the removal of paraffin or centrifugation) as appropriate with antibodies to the proteins composing the fusion protein. Alternatively, this step may be conducted by a method modified from one of the methods of detection described above by replacing the antibodies to the proteins composing the fusion protein with an antibody that recognizes the fusion point in the fusion protein. Examples of such methods include methods such as enzyme immunoassay, double antibody sandwich ELISA (enzyme-linked immunosorbent assay), fluorescent immunoassay, radioimmunoassay, Western blotting, immunohistological staining, and a combined detection method of immunoprecipitation and mass spectrometry using a monoclonal antibody or a polyclonal antibody specific for a polypeptide to be detected.

As used herein, the “fusion point” of the fusion protein means a point in which a part derived from CLDN18 gene and a part derived from ARHGAP6 gene or a part derived from ARHGAP26 gene in the polypeptide to be detected are fused. The detection using an immunohistological staining technique can be conducted, for example, by the proximity ligation assay (Nat Methods. 2006, 3 (12): 995-1000). More specifically, the presence of the polypeptide to be detected can be detected by detecting that the two antibodies recognize an identical molecule by the aforementioned techniques using an antibody that recognizes a part derived from CLDN18 gene in the polypeptide to be detected and an antibody that recognizes a part derived from ARHGAP6 gene or a part derived from ARHGAP26 gene in the polypeptide to be detected. More specifically, the detection can be conducted by i) a step of bringing an antibody (primary antibody) that recognizes a part derived from the CLDN18 gene in the polypeptide to be detected and an antibody (primary antibody) that recognizes a part derived from the ARHGAP6 gene or a part derived from the ARHGAP26 gene in the polypeptide to be detected in contact with the sample obtained from the subject; ii) a step of adding oligonucleotide-conjugated secondary antibodies that respectively bind to the primary antibodies; iii) a step of adding a ligation solution containing 2 oligonucleotides partially complementary to the oligonucleotides conjugated to the secondary antibodies and a ligase capable of ligating the 2 oligonucleotides when they come in the vicinity to form a ring structure to make ligation to cause a ligation reaction; iv) a step of extending a nucleic acid along the formed ring structure; v) a step of hybridizing a labelled oligonucleotide probe capable of hybridizing the extended nucleic acid; and vi) a step of detecting the label signal. Such detection can be conducted using a PLA probe and a reagent contained in Duolink® II reagent kit or Duolink® II Brightfield reagent kit (Olink Bioscience).

3. Method for Identification according to the Present Invention

Moreover, the “step of detecting the presence of a polynucleotide” according to the present invention and the “step of detecting the presence of a polypeptide” according to the present invention can be used in a method for identifying a subject (a patient with pancreatic cancer) for which a therapy is suitable for therapies by an ARHGAP6 inhibitor or an ARHGAP26 inhibitor and/or an agent that blocks an abnormal signal induced by the fusion gene of the CLDN18 gene and the ARHGAP6 gene or the fusion gene of the CLDN18 gene and the ARHGAP26 gene. Examples of the inhibitor of ARHGAP6 or ARHGAP26 include a function inhibitor or an expression inhibitor. Inhibiting a function refers to inhibiting a function of ARHGAP6 or ARHGAP26 as a protein. Inhibiting expression refers to inhibiting transcription and/or translation into a protein of a nucleic acid encoding the fusion gene according to the present invention or promoting the degradation of the protein. Examples of the abnormal signal induced by the fusion gene of the CLDN18 gene and the ARHGAP6 gene or the fusion gene of the CLDN18 gene and the ARHGAP26 gene include signals promoted or suppressed under the expression of these fusion genes in comparison with those under no expression of these fusion genes, for example, a signal that promotes the cell proliferation or a signal that suppresses programmed cell death. Examples of the agent that blocks an abnormal signal induced by the fusion gene of the CLDN18 gene and the ARHGAP6 gene or the fusion gene of the CLDN18 gene and the ARHGAP26 gene include an agent that down-regulates a signal that promotes the cell proliferation or an agent that removes a signal that suppresses programmed cell death.

The method for identification according to the present invention may further comprise one or more steps (for example, one or more of the additional steps A to F) listed for the method for selection according to the present invention in addition to the step of detection. Furthermore, a primer set, a probe, a probe set, and a kit for detection used in the method for selecting according to the present invention may be used. Furthermore, the method for identification according to the present invention may comprise a step of determining, when a polynucleotide to be detected or a polypeptide to be detected is detected in a sample obtained from a subject, when the amplified nucleic acid fragment is obtained with an intended size, when the amplified nucleic acid fragment comprises the nucleotide sequence of a part encoding CLDN18 and the nucleotide sequence of a part encoding ARHGAP6 or the nucleotide sequence of a part encoding ARHGAP26 in the same fragment, or when two signals, a signal from a probe designed for a part encoding CLDN18 and a signal from a probe designed for a part encoding ARHGAP6 or a part encoding ARHGAP26, are detected in the same place (the same molecule), that the subject is suitable for a therapy with an ARHGAP6 inhibitor or an ARHGAP26 inhibitor; and/or an inhibitor of a fusion gene of CLDN18 gene and ARHGAP6 gene or a fusion gene of the CLDN18 gene and the ARHGAP26 gene, or an agent that blocks an abnormal signal induced by said fusion genes and determining that the subject is not suitable for the therapy when none of the above descriptions is true.

<Primer Set, Probe, Probe Set, and Kit for Detection according to the Present Invention>

The present invention comprises a primer set, a probe, a probe set, and a kit for detection used in the method according to the present invention.

The primer set according to the present invention comprises a sense primer designed for a part encoding CLDN18 and an antisense primer designed for a part encoding ARHGAP6 or a part encoding ARHGAP26, the antisense primer consists of an oligonucleotide that hybridizes with a polynucleotide to be detected under stringent conditions (preferably under highly stringent conditions), and the sense primer consists of an oligonucleotides that hybridizes with the complementary strand to the polynucleotide to be detected under stringent conditions (preferably under highly stringent conditions).

In the primer set according to the present invention, one of the sense primer or the antisense primer may be designed so that it corresponds to a region containing a fusion point in the polynucleotide to be detected.

Specific aspects of the primer set according to the present invention include the following primer sets:

-   a primer set of a sense primer consisting of an oligonucleotide that     hybridizes with a complementary strand to the polynucleotide     consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 1     under stringent conditions and an antisense primer consisting of an     oligonucleotide that hybridizes with the polynucleotide consisting     of nucleotides at positions 751 to 3087 in SEQ ID NO: 1 under     stringent conditions; -   a primer set of a sense primer consisting of an oligonucleotide that     hybridizes with a complementary strand to the polynucleotide     consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 3     under stringent conditions and an antisense primer consisting of an     oligonucleotide that hybridizes with the polynucleotide consisting     of nucleotides at positions 751 to 2460 in SEQ ID NO: 3 under     stringent conditions; -   a primer set of a sense primer consisting of an oligonucleotide that     hybridizes with a complementary strand to the polynucleotide     consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 5     under stringent conditions and an antisense primer consisting of an     oligonucleotide that hybridizes with the polynucleotide consisting     of nucleotides at positions 751 to 2088 in SEQ ID NO: 5 under     stringent conditions; or -   a primer set of a sense primer consisting of an oligonucleotide that     hybridizes with a complementary strand to the polynucleotide     consisting of nucleotides at positions 1 to 750 in SEQ ID NO: 7     under stringent conditions and an antisense primer consisting of an     oligonucleotide that hybridizes with the polynucleotide consisting     of nucleotides at positions 751 to 1923 in SEQ ID NO: 7 under     stringent conditions.

More specific aspects of the primer set according to the present invention include the following primer sets:

-   a primer set of a sense primer consisting of at least any 16     consecutive bases between nucleotides at positions 1 to 750 in SEQ     ID NO: 1 and an antisense primer consisting of an oligonucleotide     complementary to an oligonucleotide of at least any 16 consecutive     bases between nucleotides at positions 751 to 3087 in SEQ ID NO: 1; -   a primer set of a sense primer consisting of at least any 16     consecutive bases between nucleotides at positions 1 to 750 in SEQ     ID NO: 3 and an antisense primer consisting of an oligonucleotide     complementary to an oligonucleotide of at least any 16 consecutive     bases between nucleotides at positions 751 to 2460 in SEQ ID NO: 3; -   a primer set of a sense primer consisting of at least any 16     consecutive bases between nucleotides at positions 1 to 750 in SEQ     ID NO: 5 and an antisense primer consisting of an oligonucleotide     complementary to an oligonucleotide of at least any 16 consecutive     bases between nucleotides at positions 751 to 2088 in SEQ ID NO: 5;     or -   a primer set of a sense primer consisting of at least any 16     consecutive bases between nucleotides at positions 1 to 750 in SEQ     ID NO: 7 and an antisense primer consisting of an oligonucleotide     complementary to an oligonucleotide of at least any 16 consecutive     bases between nucleotides at positions 751 to 1923 in SEQ ID NO: 7.

Preferably, the distance between the positions selected for the sense primer and the antisense primer in the primer set is 1 kb or shorter or the size of the nucleic acid fragment to be amplified with the sense primer and the antisense primer is 1 kb or smaller since the efficiency of amplification becomes lower when the size of the nucleic acid fragment to be amplified is large. Moreover, the primers according to the present invention usually have a chain length of at least 15 bases, preferably at least 16 bases, and more preferably at least 18 bases. In one embodiment, the primers have a chain length of 15 to 40 bases, preferably 16 to 24 bases, and more preferably 18 to 24 bases.

The primers included in the primer set according to the present invention are not particularly limited, but, for example, they may be prepared by a chemical synthesis process.

The probes according to the present invention and the probes included in the probe set according to the present invention comprise an oligonucleotide that hybridizes with the polynucleotide and the complementary strand to the polynucleotide to be detected under stringent conditions (preferably under highly stringent conditions). The chain length of the probes according to the present invention and the probes included in the probe set according to the present invention may be selected according to the hybridization method to be used as appropriate by a person skilled in the art, but the probes preferably have a chain length of at least 16 bases.

In one embodiment, the probes according to the present invention comprise an oligonucleotide of at least 16 bases each in the upstream and downstream of the fusion point in the polynucleotide to be detected (for specific examples, a sequence of nucleotides at positions 735 to 766 in SEQ ID NO: 1, 3, 5, or 7) or an oligonucleotide complementary thereto.

In one embodiment, the probe set according to the present invention is a probe set comprising a probe designed for a part encoding CLDN18 (for example, any part in the CLDN18 gene region in the fusion polynucleotide) and a probe designed for a part encoding ARHGAP6 or a part encoding ARHGAP26 (for example, any part in the ARHGAP6 gene or ARHGAP26 gene region in the fusion polynucleotide).

In one embodiment, the probe set according to the present invention comprises a plurality of probes designed for a part encoding CLDN18 and a plurality of probes designed for a part encoding ARHGAP6 or a part encoding ARHGAP26.

In one embodiment, the probe set according to the present invention comprises the following:

-   a plurality of flanking probe pairs (preferably 10 to 25, more     preferably 18 to 22, and more preferably 20 probe pairs) comprising     an oligonucleotide complementary to an oligonucleotide of at least     any 16 (preferably 16 to 30 bases, more preferably 18 to 25 bases)     consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO:     1 and a plurality of flanking probe pairs (preferably 10 to 25, more     preferably 18 to 22, and more preferably 20 probe pairs) comprising     an oligonucleotide complementary to an oligonucleotide of at least     any 16 (preferably 16 to 30 bases, more preferably 18 to 25 bases)     consecutive bases of nucleotides at positions 751 to 3087 in SEQ ID     NO: 1; -   a plurality of flanking probe pairs (preferably 10 to 25, more     preferably 18 to 22, and more preferably 20 probe pairs) comprising     an oligonucleotide complementary to an oligonucleotide of at least     any 16 (preferably 16 to 30 bases, more preferably 18 to 25 bases)     consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO:     3 and a plurality of flanking probe pairs (preferably 10 to 25, more     preferably 18 to 22, and more preferably 20 probe pairs) comprising     an oligonucleotide complementary to an oligonucleotide of at least     any 16 (preferably 16 to 30 bases, more preferably 18 to 25 bases)     consecutive bases of nucleotides at positions 751 to 2460 in SEQ ID     NO: 3; -   a plurality of flanking probe pairs (preferably 10 to 25, more     preferably 18 to 22, and more preferably 20 probe pairs) comprising     an oligonucleotide complementary to an oligonucleotide of at least     any 16 (preferably 16 to 30 bases, more preferably 18 to 25 bases)     consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO:     5 and a plurality of flanking probe pairs (preferably 10 to 25, more     preferably 18 to 22, and more preferably 20 probe pairs) comprising     an oligonucleotide complementary to an oligonucleotide of at least     any 16 (preferably 16 to 30 bases, more preferably 18 to 25 bases)     consecutive bases of nucleotides at positions 751 to 2088 in SEQ ID     NO: 5; or -   a plurality of flanking probe pairs (preferably 10 to 25, more     preferably 18 to 22, and more preferably 20 probe pairs) comprising     an oligonucleotide complementary to an oligonucleotide of at least     any 16 (preferably 16 to 30 bases, more preferably 18 to 25 bases)     consecutive bases of nucleotides at positions 1 to 750 in SEQ ID NO:     7 and a plurality of flanking probe pairs (preferably 10 to 25, more     preferably 18 to 22, and more preferably 20 probe pairs) comprising     an oligonucleotide complementary to an oligonucleotide of at least     any 16 (preferably 16 to 30 bases, more preferably 18 to 25 bases)     consecutive bases of nucleotides at positions 751 to 1923 in SEQ ID     NO: 7.

The probes according to the present invention and the probes included in the probe set according to the present invention are not particularly limited, but, for example, they may be prepared by a chemical synthesis process.

The present invention comprises a kit for detection comprising a primer set according to the present invention, a probe according to the present invention, or a probe set according to the present invention. The kit for detection according to the present invention may comprise, in addition to the primer set according to the present invention, the probe according to the present invention, or the probe set according to the present invention, a component to be used with the primer set, probe, or probe set for detecting the polynucleotide to be detected, such as a reagent for amplifying a signal of the hybridization.

The present invention also comprises a kit for detection for detecting a polypeptide to be detected. Preferably, the kit for detection comprises an antibody (primary antibody) that recognizes a part derived from CLDN18 gene in the polypeptide to be detected and an antibody (primary antibody) that recognizes a part derived from ARHGAP6 gene or a part derived from ARHGAP26 gene in the polypeptide to be detected. More preferably, the kit may comprise oligonucleotide-conjugated secondary antibodies that respectively bind to the primary antibodies, 2 oligonucleotides partially complementary to the oligonucleotides conjugated to the secondary antibodies, a ligase capable of ligating the 2 oligonucleotides when they are in the vicinity to form a ring structure, a polymerase capable of extending a nucleic acid along the ring structure, and a labelled oligonucleotide probe.

The primer set, probe, probe set, and kit for detection according to the present invention may be used in the method for selection and the method for identifying a patient according to the present invention. In one embodiment, in relation to the primer set, probe, probe set, and kit for detection according to the present invention, a subject is a subject suspected to have pancreatic cancer or a subject having pancreatic cancer.

EXAMPLES

Unless otherwise specified, the present invention can be performed according to known methods. Further, the present invention can be performed, when using commercially available reagents, kits, or the like, according to instructions or protocols of the commercially available products.

Example 1 Detection of Polynucleotides containing a Fusion Point of CLDN18-ARHGAP6/26 Fusion Gene

The ascites collected from 26 pancreatic cancer patients in National Cancer Center Hospital and Kanamecho Hospital under the comprehensive consent were suspended in ISOGEN (NIPPON GENE CO., LTD.) according to the standard protocol of the reagent, and stored. From the conserved solution, total RNA was purified. Thereafter, reverse transcription reaction was performed with a High-Capacity cDNA Reverse Transcription Kit (Thermo Fisher Scientific K.K.) using 1 μg of the total RNA as a template according to the standard protocol of the reagent to synthesize cDNA. Next, PCR reaction (2 minutes at 94° C. followed by 15 seconds at 94° C., 15 seconds at 55° C. and 1 minute at 68° C., 30 or 35 cycles) was performed with DNA polymerase (AccuPrime™ Taq DNA Polymerase; Thermo Fisher Scientific K.K.) using the cDNA as a template (100 ng in terms of total RNA) with a reaction volume of 20 μL to amplify a region containing fusion point of the fusion gene. In this PCR, CLD-ARH_C6A12_partial fwd01 set forth in SEQ ID NO: 9, and CLDN18-ARHGAP6_CDS_R893 set forth in SEQ ID NO: 10 were used as primers for the amplification of CLDN18-ARHGAP6. Meanwhile, CLD-ARH_C6A12_partial fwd01 described above and CLD-ARH_C6A12_partial rev01 set forth in SEQ ID NO: 11 were used as primers for the amplification of CLDN18-ARHGAP26. Thereafter, nested PCR reaction was performed using 2 μL of the obtained PCR products as a template under the same conditions as described above. In this nested PCR, CLD-ARH_C6A12_partial fwd02 set forth in SEQ ID NO: 12 and CLDN18-ARHGAP6_CDS_R815 set forth in SEQ ID NO: 13 were used as primers for the amplification of CLDN18-ARHGAP6. Meanwhile, CLD-ARH_C6A12_partial fwd02 described above and CLD-ARH_C6A12_partial rev02 set forth in SEQ ID NO: 14 were used as primers for the amplification of CLDN18-ARHGAP26. It is expected that the use of these primers yield PCR products having nucleotide length of about 300 bp when CLDN18-ARHGAP6 fusion genes exist, and PCR products having a nucleotide length of about 500 bp when CLDN18-ARHGAP26 fusion genes exist, respectively. Further, to confirm that the amount of template cDNA is the same level, PCR reaction (2 minutes at 94° C. followed by 15 seconds at 94° C., 15 seconds at 55° C. and 1 minute at 68° C., 25 cycles was performed for ACTB (β-actin) using ACTB_F2 set forth in SEQ ID NO: 15 and ACTB_R2 set forth in SEQ ID NO: 16 as primers, and the same DNA polymerase as described above. Thereafter, the obtained PCR products were subjected to electrophoresis in 2% agarose gel (Lonza Ltd.). From the result of electrophoresis, PCR products having almost the same nucleotide length as described above was confirmed for CLDN18-ARHGAP6 in all of the 9 specimens (among the 9 specimens, 1 specimen (No.193) is shown in FIG. 1). PCR products having almost the same nucleotide length as described above was also confirmed for CLDN18-ARHGAP26 in 1 specimen (No. 164) (FIG. 2).

Next, the PCR products of all the specimens of which the amplification product in the PCR reaction had been confirmed were cloned to cloning vectors (TOPO TA Cloning® Kit; Thermo Fisher Scientific K.K.). Sequence of the inserted nucleotides were determined using DNA sequencing service provided by Eurofins Genomics K.K. From the result, it was confirmed with respect to CLDN18-ARHGAP6 that the existence of the transcripts in all of the 9 specimens in which the amplification product had been confirmed, in which a nucleotide at position 803 in CLDN18 (NCBI accession number: NM_001002026.2) and a nucleotide at position 1462 in ARHGAP6 (NCBI accession number: NM_013427.2 or NCBI accession number: NM_006125.2) were fused , which had already been reported in gastric cancer. FIG. 3 shows the sequence of total 80 nucleotides, each 40 nucleotides upstream and downstream of the fusion point. It was also confirmed with respect to CLDN18-ARHGAP26 that the existence of the transcripts in one specimen in which the amplification products had been confirmed, in which a nucleotide at position 803 in CLDN18 (NCBI accession number: NM_001002026.2) and a nucleotide at position 1143 in ARHGAP26 (NCBI accession number: NM_015071.4 or NCBI accession number: NM_001135608.1) were fused. FIG. 3 shows the sequence of total 80 nucleotides, each 40 nucleotides upstream and downstream of the fusion point. From the results, it was showed that CLDN18-ARHGAP6/26 fusion genes are present not only in gastric cancer where the fusion gene has already been reported, but also in pancreatic cancer.

Reference Example 1 Detection of CLDN18-ARHGAP26 Fusion Gene in an Established Cell Line

Total RNA were prepared from total four cell lines: a gastric cancer cell line NSC-47C established by Department of Biomarker Discovery of National Cancer Center Research Institute in Japan; a gastric cancer cell line HSC-39 (obtained from Department of Animal Experimentation of National Cancer Center Research Institute in Japan); a gastric cancer cell line KATO-III (JCRB0611, JCRB Cell Bank); and a gastric cancer cell line NSC-9C established by Biomarker Discovery Department of National Cancer Center Japan Research Institute. For these total RNAs, reverse transcription was performed to synthesize cDNA using SuperScript® III First-Strand Synthesis System (Thermo Fisher Scientific K.K.), in which reverse transcriptase was included, and oligo (dT) primers (oligo(dT) 20 primer; Thermo Fisher Scientific K.K.) according to the standard protocols of the reagent.

Next, using the obtained cDNA as a template (200 ng in terms of total RNA), PCR reaction (2 minutes at 94° C. followed by 15 seconds at 94° C. 15 seconds at 55° C. and 1 minute at 68° C. 30 cycles) was performed using CLD-ARH_C6A12_partial fwd02 set forth in SEQ ID NO: 12 and CLD-ARH_C6A12_partial rev02 set forth in SEQ ID NO: 14, which were used in Example 1, as primers, and DNA polymerase (AccuPrime™ Taq DNA Polymerase; Thermo Fisher Scientific K.K.). After the PCR reaction, the obtained PCR products were subjected to electrophoresis in 2% agarose gel (Lonza Ltd.). From the results, it was confirmed that PCR products at the near the expected molecular weight (about 500 bp) was yielded only in the gastric cancer cell line NSC-47C (FIG. 4). Meanwhile, to confirm that the amount of the template cDNA was the same level, PCR reaction (2 minutes at 94° C. followed by 15 seconds at 94° C., 15 seconds at 55° C. and 1 minute at 68° C. 25 cycles was performed for ACTB (β-actin) using ACTB_F2 set forth in SEQ ID NO: 15 and ACTB_R2 set forth in SEQ ID NO: 16 as primers and DNA polymerase as described above. From the results, tt was shown that the gastric cancer cell line NSC-47C expressed CLDN18-ARHGAP26 endogenously.

Reference Example 2 Evaluation of Suppressive Effect of siRNA to CLDN18-ARHGAP26 Fusion Protein in a Gastric Cancer Cell Line Expressing CLDN18-ARHGAP26 Fusion Gene, and Evaluation of the Viability of the Cell Line under the same Condition

The gastric cancer cell line NSC-47C that endogenously expresses CLDN18-ARHGAP26 fusion genes as shown in Reference Example 1 was cultured in a RPMI-1640 medium (Wako Pure Chemical Industries, Ltd.) containing 10% fetal bovine serum (Thermo Fisher Scientific K.K.), and then intended siRNA molecules were introduced into the cultured cells according to the standard protocol of DharmaFECT™1, a gene transfection reagent (GE HealthCare, Inc.) (herein also referred to as “siRNA treatment”). More specifically, the gastric cancer cells were seeded into 6-well plates (140675, Nunc) at 2×10⁵ cells per well, and then siRNAs targeting CLDN18 (s27688, Thermo Fisher Scientific K.K.), siRNAs targeting ARHGAP26 (s23013 and s23015, Thermo Fisher Scientific K.K.), and control siRNAs (AM4611, Thermo Fisher Scientific K.K.) were added onto the cells respectively so as to be 75 pmol (final concentration of 75 nM) and then cultured at 37° C. under 5% CO₂ condition for 72 hours. Among the siRNAs targeting ARHGAP26, s23013 is expected to target wild-type genes and also the fusion genes, while s23015 is expected to target only wild-type and not the fusion genes from their sequences. Furthermore, the siRNAs targeting CLDN18 is expected to target wild-type genes and also the fusion genes from the sequence. Hereinafter, the group treated with control siRNAs is referred to as Control siRNA group, the group treated with the siRNA targeting CLDN18 is referred to as CLDN18 siRNA group, and the group treated with s23013, one of siRNAs targeting ARHGAP26, is referred to as ARHGAP26 siRNA (Wild & Fusion) group, and the group treated with s23015, the other siRNA targeting ARHGAP26, is referred to as ARHGAP26 siRNA (Wild) group, respectively.

Suppressive effect by introducing siRNAs to the expression level of CLDN18-ARHGAP26 fusion protein was evaluated by Western blotting. Specifically, cell pellets after the culture were washed with PBS, lysed with 120 μL of Cell Lysis Buffer supplied in RhoA Pull-down Activation Assay Biochem Kit™ (Cytoskeleton, Inc.), and centrifuged at 4° C., 10000×g for 1 minute to collect supernatants. The supernatants were used as a protein extract. The protein concentration of this protein extract was measured by Protein Quantification Assay (MACHEREY-NAGEL GmbH & Co. KG). Thereafter, ARHGAP26 was detected by the following experiment. Specifically, the protein extract was loaded onto Novex™ WedgeWell™ 4-20% Tris-Glycine Gel (Thermo Fisher Scientific K.K.) at 20 μg per lane, and subjected to gel electrophoresis at 225 V for 25 minutes, then transferred to a PVDF membrane (Merck KGaA) using Trans-Blot® Turbo™ Blotting System (Bio-Rad Laboratories, Inc.) according to the standard protocol. After the transfer, the membrane was blocked with PBS containing 5% of ECL Blocking Agent (GE HealthCare, Inc.) (hereinafter, referred to as a blocking buffer) at room temperature for two hours. Then, the membrane was shaken in a primary antibody solution prepared by diluting anti ARHGAP26 antibody (HPA035107, Sigma-Aldrich Co. LLC) with the blocking buffer at a ratio of 1:500, and incubated at 4° C., overnight. After washing with PBS containing 0.05% Tween® 20 (Wako Pure Chemical Industries, Ltd.) (hereinafter, referred to as a washing buffer), the membrane was shaken in a secondary antibody solution prepared by diluting HRP-labeled anti-rabbit antibody (P0399, Dako) with the blocking buffer at a ratio of 1:3000, and incubated at room temperature for one hour. After washing with the washing buffer, Pierce Western Blotting Substrate Plus (Thermo Fisher Scientific K.K.) was added onto the membrane, and chemiluminescence signal on the membrane was detected using ImageQuant™ LAS 4000 mini (GE HealthCare, Inc.) (“detection of ARHGAP26”). For detection of β-actin, the membrane after the detection of ARHGAP26 was incubated with 14 mL of Restore™ PLUS Western Blot Stripping Buffer (Thermo Fisher Scientific K.K.) at room temperature for 15 minutes, washed with the washing buffer, and subjected to the same experimental procedure as described above. The antibody used in this experiment was an anti-β-actin antibody (4967, Cell Signaling Technology, Inc.), and the dilution ratio was 1:3000. As the result of Western blot, it was confirmed that the expression level of CLDN18-ARHGAP26 fusion protein was suppressed in the CLDN18 siRNA group and the ARHGAP26 siRNA (Wild & Fusion) group (FIG. 5).

Next, in order to evaluate the effect of CLDN18-ARHGAP26 fusion gene on the viability of cancer cells, siRNA treatment was performed to the gastric cancer cell line NSC-47C under the same conditions as described above. After 24 hours, medium was exchanged to RPMI-1640 medium containing 10% fetal bovine serum, and then 100 μL from each group were seeded respectively into 3 wells of a 96-well plate (167008, Nunc) so as to be 1000 cells per well. Wells only added RPMI-1640 medium containing 10% fetal bovine serum without cells were also prepared as a control (hereinafter, referred to as a medium group). Total three plates were prepared. The number of viable cells was measured at the day of cell seeding (referred to as day 1), at the additional 48 hours (referred to as day 3) and 120 hours (referred to as day 6) cultured at 37° C. under 5% CO₂ condition, respectively. The number of viable cells was determined by measuring luminescence intensity using Synergy™ H1 microplate reader (Biotech) with the same settings of the parameters following the standard protocol of CellTiter-Glo® 3D Cell Viability Assay (Promega Corporation). The viability in each siRNA group was calculated from a value obtained by subtracting the luminescence intensity of the medium group from the intensity in each siRNA group (hereinafter, referred to as correction value), where the correction value of day 1 in each siRNA group was determined as 1. The standard deviation was used as an error range. The student's t-test (two-sided test) was used f for the significance test, and it was determined significant when p<0.01.

As a result, it was found that, in the gastric cancer cell line NSC-47C which expresses CLDN18-ARHGAP26 fusion genes endogenously, the viability of the cells were attenuated significantly by suppressing the expression of CLDN18-ARHGAP26 fusion proteins (FIG. 6). Therefore, it was revealed that the suppression of the expression level of CLDN18-ARHGAP26 fusion genes in cancer cells that endogenously express the fusion genes inhibits the proliferation and/or attenuates the viability of the cancer cells.

From the above, it was revealed that CLDN18-ARHGAP26 is involved in tumor promoting ability. Meanwhile, it is expected that CLDN18-ARHGAP6 also relates to the tumor promoting ability, because ARHGAP6 is known to have the same function (enhancement of GTP hydrolase activity of RhoA) as ARHGAP26, and the ARHGAP6-derived sequence constituting the CLDN18-ARHGAP6 fusion gene also retains the Rho-GAP domain responsible for enhancing the function of GTP hydrolase activity same as the ARHGAP26-derived sequence constituting the CLDN18-ARHGAP26 fusion genes.

INDUSTRIAL APPLICABILITY

The method according to the present invention is for selecting subjects positive for the fusion genes of CLDN18 gene and ARHGAP6 gene, or subjects positive for the fusion genes of CLDN18 gene and ARHGAP26 gene, and the method is expected to be useful to identify subjects to be eligible for therapies with drugs. Moreover, the primer set, the probe, the probe set, and the kit for the detection according to the present invention can be used in the method according to the present invention. 

1. A method for detecting a fusion gene of a Claudin-18 (CLDN18) gene and a Rho GTPase Activating Protein 6 (ARHGAP6) gene, comprising detecting the presence of a polynucleotide encoding a fusion protein of CLDN18 and ARHGAP6 comprising an amino acid sequence having 90% or more identity with SEQ ID NO: 2 or SEQ ID NO: 4 in a sample obtained from a subject suspected to have pancreatic cancer or a subject having pancreatic cancer, wherein detecting comprises amplifying the polynucleotide in the sample using a primer set comprising a first primer corresponding to a section of the CLDN18 gene and a second primer corresponding to a section of the ARHGAP6 gene.
 2. The method according to claim 1, wherein the polynucleotide encodes a polypeptide comprising SEQ ID NO: 2 or SEQ ID NO:
 4. 3. The method according to claim 1, wherein the polynucleotide is: a polynucleotide comprising a nucleotide sequence having 90% or more identity with SEQ ID NO: 19; or a polynucleotide comprising a nucleotide sequence of SEQ ID NO:
 19. 4. The method according to claim 3, wherein the polynucleotide comprises SEQ ID NO: 18, 19, 20, or
 21. 5. The method according to claim 1, wherein the method comprises hybridizing a probe to the polynucleotide.
 6. The method according to claim 1, wherein the first primer is a sense primer and the second primer is an antisense primer, and wherein the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide under stringent conditions and the sense primer consists of an oligonucleotide that hybridizes with a complementary strand to the polynucleotide under stringent conditions.
 7. The method according to claim 1, wherein the sample is body fluid or body cavity lavage fluid.
 8. The method according to claim 1, wherein the sample is ascites or peritoneal cavity lavage fluid.
 9. The method of claim 1 further comprising identifying the subject as suitable for a therapy with an ARHGAP6 inhibitor and/or an agent that blocks an abnormal signal induced by the fusion gene of the CLDN18 gene and the ARHGAP6 gene if the fusion gene is present in the sample. 